1. Field of the Invention
The present invention relates to a toner and a developer for use in copiers, facsimiles and printers using electrophotographic image forming methods.
2. Discussion of the Background
The electrophotographic image forming method includes a charging process charging a surface of a photoreceptor which is an image bearer with an electric discharge, an irradiating process irradiating the charged surface of the photoreceptor to form an electrostatic latent image, a developing process developing the electrostatic latent image formed on the surface of the photoreceptor with a toner to form a toner image, a transfer process transferring the toner image on the surface of the photoreceptor onto a surface of a transfer body, a fixing process fixing the toner image on the surface of the transfer body and a cleaning process removing the toner remaining on the surface of the image bearer after the transfer process.
Recently, color image forming apparatuses using the electrophotographic image forming method are widely used, and digitalized images are available with ease and printed images are required to have higher image definitions. While higher image resolution and gradient are studied, the toner visualizing the latent image is studied to have further sphericity and smaller particle diameter to form a high definition images. As the toner prepared by pulverizing methods has a limit of these properties, polymerized toners prepared by suspension polymerizing methods, emulsification polymerizing methods and dispersion polymerizing methods capable of conglobating the toner and making the toner have a small particle diameter are being used.
The toner having a shape close to a true sphere is easily affected by a line of electric force in an electrostatic developing method and is faithfully developed along the line of electric force of an electrostatic latent image on a photoreceptor. When a minute latent image dot is reproduced, the toner are precisely and uniformly located to have a high thin line reproducibility. In an electrostatic transfer method, as the toner has a smooth surface and a good powder fluidity, the toner particles less adhere each other and to the photoreceptor, and therefore the toner is easily affected by a line of electric force and is faithfully transferred along the line of electric force, i.e., the toner has a high transferability.
However, the toner having a shape close to a true sphere has a smaller surface area than an amorphous toner, i.e., has less surface area which can effectively used for frictional charge by a magnetic carrier and friction charging members such as developer regulating members. The spheric toner easily slip on a surface of the friction charging member and charged speed and level thereof decrease, and therefore a specific amount or more of a charge controlling agent is needed therefor.
In addition, as the toner having a smaller particle diameter to improve minute dot reproducibility has a lower friction chargeability, it is essential for the toner to have chargeability, developability and transferability.
Japanese Laid-Open Patent Publications Nos. 9-179331, 10-142835 and 11-327197 discloses various methods of controlling the shape of a spheric toner and a toner having a small particle diameter. Shape factors SF-1 and SF-2 are mostly used as indices to represent the shape of a toner. The SF-1 is an index representing roundness of the toner particle and the SF-2 is an index representing concavity and convexity thereof. Either of the SF-1 and SF-2 or both thereof are specified to control the shape of a toner and even a spheric toner or a toner having a small particle diameter is tried to have the chargeability, developability, transferability and cleanability.
Japanese Laid-Open Patent Publication No. 2001-51444 specifies a surface area ratio having the following formula as well as the shape factors of the toner particles:surface area ratio=ρ×D50p×S wherein ρ is a specific gravity of the toner particle (g/m3), D50p is a number-average particle diameter (m) thereof and S is a BET specific surface area (m3/g) thereof. The surface area ratio represents the concavity and convexity of the toner particle in a different scale from that of the shape factor. When the surface area ratio is greater than the specified range, the concavity and convexity on a surface of the toner particle become large and an external additive externally added thereto enters the concave with time, and therefore the chargeability and transferability cannot be maintained for a long time.
As mentioned above, trials to improve the chargeability, developability, transferability and cleanability of the toner are made by controlling the shape of the toner particle. However, any of the trials roughly sees the surface shape of the toner particle and does not microscopically see the concavity and convexity.
Because of these reasons, a need exists for a spheric toner having a small particle diameter, which has good chargeability, developability and transferability.